1. Field of the Invention
This invention relates to a multi-stage tube forging method for disproportionally enlarging an end section of an aluminum alloy tube of a bicycle frame part.
2. Description of the Related Art
FIG. 1 illustrates a conventional method of forming a bicycle tube that includes the steps of cutting a hollow rigid tube 1 having a predetermined length, and placing the tube 1 thus cut in a tube forming machine. During the tube forming operation, the middle section of the tube 1 is clamped by a positioning clamp seat 11 of the tube forming machine, with a pair of mandrels 13 inserted respectively and movably into two opposite ends of the tube 1 and a pair of mold components 12 sleeved respectively and movably on the ends of the tube 1. The inner diameter and wall thickness of the tube 1 are changed along the length of the tube 1 as a result of the movement of the mold components 12 and the pair of mandrels 13.
U.S. Pat. No. 6,453,714 describes a method for forming an eccentrically expanded pipe that includes the steps of coaxially expanding a portion of a base pipe 2 at least one time by using a first expander punch (not shown) to form an expanded portion 21, an intermediate tapering portion 22 and a neck portion 23, and as shown in FIG. 2, decentering the expanded portion 21 and the neck portion 23 relative to one another by using a second expander punch 24 having a diameter larger than that of the first expander punch.
The abovementioned conventional methods are disadvantageous in that the body of either the tube 1 or the base pipe 2 is subjected to a relatively large internal stress therein. In addition, if the tube 1 or the base pipe 2 is made from aluminum alloy, the same tends to break during the enlarging operation.